This is a diagonal fisheye. This normally means 180 degree field of view on the image diagonal. The sample I got is Canon mount version. Due to the slightly smaller Canon 1.6x crop compared to the designed 1.5x crop, the claimed field of view is reduced to only 167 degrees. That's still pretty wide.

Lens body is made from plastic, which is sturdy enough but the finish feels cheap in places like the cap. The mount is made from metal. There are no electrical contacts on the lens, and operation is fully manual. The rear of the lens is flush with the mount and therefore should be physically compatible with a full frame body.

An aperture ring has marked steps on it going from f/3.5 to f/22. It clicks in place at these settings and once between each pair, except between f/3.5 and f/5.6 which has no more steps. The focus ring is nicely damped and marked from 0.3m to infinity, taking about 1/3 of a rotation to do so. The lens does extend when focusing closer, but you have to look for it. It is in the region of 1mm difference. The front doesn't rotate, although that's a rather moot point since you can't fit a conventional filter over the glass bulge on the front. If you really want to use a filter, the only possibility might be to tape a thin film on the back of the lens - at your own risk. The lens hood is built into the body and fixed in place. This will be the only protection for the front element while in use.

Observationally there are 6 rounded aperture blades, giving a bloated hexagonal look in mid range. Then again bokeh isn't likely to be a major selling point given the extreme wide angle. More likely this lens would be used approximately hyperfocal so practically everything is relatively in focus.

I've only managed to fire off a few shots with it while at work. Rather dark and wet outside so unless I find inspiration at home this will have to do for now.

I love the way it "folds" back space away from the center, but haven't got a clue how to use it yet. Should be fun finding out

Taken the other night. Stars were out, but hand held 1s exposures weren't working out. When I got home I stuck it in my garden on a tripod which of course helps a lot, but cloud had moved in.

Got this on the way back from ducking today. Not bad flare control considering I'm pointing nearly directly at the sun. Just the tiny splodge lower left of center, and the arc top right. I like the 6 pointed star effect from the aperture.

I found focus to be futile. Played safe going a bit longer than hyperfocal - need to work out what I should be using. Doesn't feel sharp wide open at f/3.5, and in that condition the focus seemed better when set too close. Found myself using f/8 to f/11 most of the time to help with the exposure range. The 50D I used it with didn't seem to meter accurately with this stopped down, generally resulting in over-exposure. Had to use about -1EV compensation.

I put the subject exactly 50cm in front of the sensor plane. Following are 100% lossless crops from camera jpeg although the host is known to reprocess the image so it isn't strictly lossless any more.

This is the output at f/3.5 with the best focus I can get at maximum zoom in live view. It was set to about 1/3 of the way between 30cm and 40cm marking.

This is what you get if you set it to the 50cm mark on the lens. Not shown, but the background here was sharper (in focus) compared to the earlier shot.

Given that, it seems to confirm my suspicions from yesterday that setting the focus closer than scale suggests gave a sharper image.

There is a slight shift in perspective noticeable switching between the two, probably due to the lens extension when close focusing.

Next is a selection of shots at various apertures. I set the camera to aperture priority mode with spot metering centered right on the subject. As well as the relative sharpness, this illustrates the metering accuracy, or lack thereof, when using it with the 50D. No exposure compensation applied. Note the lighting was not controlled (sun with varying cloud) so do not pay too much attention to the actual shutter time.

f/3.5

f/5.6

f/8

f/11

f/16

f/22

There's no escaping it, at f/3.5 the lens gives a rather soft and glowy image, even allowing for that apparent sharpness is impacted by contrast, and in this case the varying exposure. This improves in the mid range of f/5.6 to f/11 before diffraction kicks in and softens the image again.

Exposure wise, remember this target is mostly white, so the metering should set this to the mid-grey level or thereabouts. At f/3.5 the histogram is in the left half so about right. In the region around f/5.6 to f/11 the histogram is about 2/3 to 3/4 to the right, which is in line with the overexposure I was seeing in real world shooting.

Using this guide I adjusted the focus scale today. Loosen three screws, turn the ring to align focus distance with the scale, tighten again. What the guide doesn't say is that you mustn't over-tighten the screws as it grips too far and makes focus very stiff. Also I get some rubbing feeling between 0.3 and 0.5m setting. Previously this is the range you couldn't access due to misaligned focus scale.

I set it for infinity focus and checked it at 30cm which seemed good too. As a final test, I put the camera a random distance away from the target, and set the focus as best as I could. The scale ended up between 3ft and 1m. When I got the tape measure out, it was just under 1m! Perfect.

Now why couldn't they do this to the lens out of the factory...

I'm going away for a few days so hope to get something interesting with this.

I didn't get to use it much on that trip after all... other than the one in this thread.

Yes, it is fisheye. I'm not sure if there is a formal definition of one, but it's ultra-wide angle and not distortion corrected. Due to the Canon crop factor it is a little less than 180 degrees FoV but not far off.